Justice failed Trayvon Martin the night he was killed. We should be appalled and outraged, but perhaps not surprised, that it failed him again Saturday night with a verdict setting his killer free.

Our society considers young black men to be dangerous, interchangeable, expendable, guilty until proven innocent. This is the conversation about race that we desperately need to have -- but probably, as in the past, will try our best to avoid.

George Zimmerman’s acquittal was set in motion on Feb. 26, 2012, before Martin’s body was cold. When Sanford, Fla., police arrived on the scene, they encountered a grown man who acknowledged killing an unarmed 17-year-old boy. They did not arrest the man or test him for drug or alcohol use. They conducted a less-than-energetic search for forensic evidence. They hardly bothered to look for witnesses.

Only a national outcry forced authorities to investigate the killing seriously. Even after six weeks, evidence was found to justify arresting Zimmerman, charging him with second-degree murder and putting him on trial. But the chance of dispassionately and definitively establishing what happened that night was probably lost. The only complete narrative of what transpired was Zimmerman’s.

Jurors knew that Zimmerman was an overeager would-be cop, a self-appointed guardian of the neighborhood who carried a loaded gun. They were told that he profiled Martin -- young, black, hooded sweatshirt -- as a criminal. They heard that he stalked Martin despite the advice of a 911 operator; that the stalking led to a confrontation; and that, in the confrontation, Zimmerman fatally shot Martin in the chest.

The jurors also knew that Martin was carrying only a bag of candy and a soft drink. They knew that Martin was walking from a 7-Eleven to the home of his father’s girlfriend when he noticed a strange man in an SUV following him.

To me, and to many who watched the trial, the fact that Zimmerman recklessly initiated the tragic encounter was enough to establish, at a minimum, guilt of manslaughter. The six women on the jury disagreed.

Those jurors also knew that Martin, at the time of his death, was just three weeks past his 17th birthday. But black boys in this country are not allowed to be children. They are assumed to be men, and to be full of menace.

I don’t know if the jury, which included no African-Americans, consciously or unconsciously bought into this racist way of thinking -- there’s really no other word. But it hardly matters, because police and prosecutors initially did.

The assumption underlying their ho-hum approach to the case was that Zimmerman had the right to self-defense but Martin -- young, male, black -- did not. The assumption was that Zimmerman would fear for his life in a hand-to-hand struggle but Martin -- young, male, black -- would not.

Page 2 of 2 - If anyone wonders why African-Americans feel so passionately about this case, it’s because we know that our 17-year-old sons are boys, not men. It’s because we know their adolescent bravura is just that -- an imitation of manhood, not the real thing.

We know how frightened our sons would be, walking home alone on a rainy night and realizing they were being followed. We know how torn they would be between a child’s fear and a child’s immature idea of manly behavior. We know how they would struggle to decide the right course of action, flight or fight.

And we know that a skinny boy armed only with candy, no matter how big and bad he tries to seem, does not pose a mortal threat to a healthy adult man who outweighs him by 50 pounds and has had martial arts training (even if the lessons were mostly a waste of money). We know that the boy may well have threatened the man’s pride, but likely not his life. How many murders-by-sidewalk have you heard of recently? Or ever?

The conversation we need to have is about how black men, even black boys, are denied the right to be young, to be vulnerable, to make mistakes. We need to talk about why, for example, black men are no more likely than white men to smoke marijuana but nearly four times as likely to be arrested for it -- and condemned to a dead-end cycle of incarceration and unemployment. I call this racism. What do you call it?

Trayvon Martin was fighting more than George Zimmerman that night. He was up against prejudices as old as American history, and he never had a chance.